Natural convection internal heat exchangers, single and double walled, for existing hot water tanks. This invention pertains to the input or extraction of heat from an existing storage tank, which is plumbed into an existing “city” pressure water system for home or industrial use. Heat is transferred from a heated or cooled fluid in a separate heat transfer loop, which is not pressurized by “city” water pressure and must be separated from it by one or two walls to prevent contamination.
If the heated or cooled fluid is non-toxic, the single wall isolation from potable water is acceptable. If the heated or cooled fluid is toxic, then double wall isolation from potable water is needed. This invention allows an existing hot water tank to be adapted for heating by a solar energy heated fluid or a heat pump heated fluid, without modifying the existing tank except by placing an adapter between it and an output water line at “city” pressure. This use of an existing tank can help to reduce the cost of solar hot water heating and heat pump hot water heating. The adapter could also be placed on the coldwater inlet, water drain port or other water tank standard port.